The present invention relates generally to a servo system in a disc drive. More particularly, the present invention relates to a disc drive having an enhanced dual stage servo control system.
A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by an air bearing which flies above each disc. The transducers and the air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the discs and to store information on the discs.
An actuator typically operates within a closed-loop servo system. The actuator typically includes an actuator arm that supports a flexure of flexure assembly which, in turn, supports the data head. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations. The time when the data head is near a target track and fine tuning its position just prior to commencing track following is referred to as track settle or seek settle.
Information is typically stored on the discs by providing a write signal to the data head to encode information on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the servo actuator so that the data head flies above the disc, sensing the on the disc, and generating a read signal based on that information. The read signal is then decoded by the drive controller to recover the data represented by the information stored on a disc, and consequently represented in the read signal provided by the data head.
Accurate positioning of the data head over a track on the disc is of great importance in writing data to the disc and reading data from the disc.
In prior systems, servo operations were accomplished based on a dedicated servo head. In a dedicated servo type of system, servo information is all written to one dedicated surface of a disc in the disc drive. All of the heads in the disc drive are mechanically coupled to the servo head which is used to access the servo information. Thus, all of the heads in the dedicated servo disc drive are positioned based on the servo information read from the servo surface. This type of system allows the disc drive to conveniently execute parallel read and write operations. In other words, with appropriate circuitry in the drive controller, read and write operations can be executed in parallel using a plurality of the data heads mounted on the actuator, the data heads being simultaneously positioned based on the servo information read from the dedicated servo surface.
However, track densities on magnetic discs have been increasing for many years. Increased track densities on the magnetic disc require more accurate and higher resolution positioning. The mechanical offset between heads in a dedicated servo system can exceed one track width. Thus, the industry has seen a tendency to shift to embedded servo information in certain applications.
In an embedded servo system, servo information is embedded on each track on each surface of every disc. Thus, each data head returns a position signal independently of the other data heads. Therefore, the servo actuator is used to position each individual data head while that particular data head is accessing information on the disc surface. The positioning is accomplished using the embedded servo data for the track over which the data head is then moving.
Prior conventional servo controllers have included proportional-integral-derivative (PID) controllers which are composed of two components: an observer and a regulator. The observer receives input position information each time a servo sector is crossed, and estimates position and velocity. The regulator then provides feedback on the observed signals. In a seek mode, the regulator typically zeros the error between a reference velocity trajectory and the observed velocity. In track following mode, the regulator zeros the error between the desired track position and the observed track position. The regulator controls according to a PID control technique.
However, PID controllers can be difficult to implement in all disc drive applications. For example, it may be desirable to provide microactuators between the flexure assembly and the transducer or slider assembly or on the actuator arm or on the suspension or flexure assembly. Where microactuators are provided, the servo actuator system might evolve from a single input single output (SISO) system where the input is an error signal and the output is a voice coil current signal, to a multiple input multiple output (MIMO) system which receives a variety of inputs from the microactuators and provides a position output signal to the voice coil motor and each of the microactuators. Such a system could also be controlled by simply decentralizing a PID controller so one target track input is received, but two outputs are provided, one for the voice coil motor and one for the microactuator. This may present problems. For example, it is difficult to control both actuators with a single controller while maintaining desired gain and stability for both control loops during track follow as well as track seek and settle. While Messner has discussed a system in Schroeck and Messner, On Controller Design For Linear Time-Invariant Dual-Input Single-Output Systems this is only directed to accommodating track follow with a PQ controller.
Further, features and benefits will become apparent upon a review of the attached drawings and accompanying detailed description.
The present invention includes a servo control system for controlling a dual stage actuator in a disc drive. The disc drive includes a data head for accessing information on a disc surface, and the dual stage actuator includes a coarse actuator and at least one fine actuator. The control system includes a fine actuator driver and a coarse actuator driver for driving the fine and coarse actuators, respectively. The control system also includes an observer which receives the coarse and fine actuator input signals and estimates a response of the coarse and fine actuators. The estimated response is used in driving the coarse and fine actuators. A gain boost circuit is coupled to the fine and coarse actuator drivers and introduces a gain boost to reduce track following error in a desired frequency range.